1. Field of the Invention
The present invention relates to a method for thermally investigating a material, wherein the material is exposed to a time controlled temperature profile, and a signal representative of a resulting thermal response is measured and evaluated, said temperature profile including a succession in time of dynamic segments wherein said temperature is varied for an associated period of time, and of non-dynamic segments wherein a variation of said temperature is small in relation to said temperature variation in said dynamic segments for an associated period of time, each of said dynamic segments being interposed between two successive ones of said non-dynamic segments, said evaluation including the use of response signal values measured during time periods associated with said dynamic segments to derive a heat capacity related information from said measured response signal.
2. Description of the Related Art
A known method of this type (S. C. Mraw and D. F. Naas, J. Chem. Thermodynamics 1979, 11, 567-584) is differential scanning calorimetry (DSC), wherein the response signal is indicative of a differential heat flow which is determined as the difference between heat flows associated with a sample material and a reference material. In this known method, all time periods associated with the non-dynamic segments are of uniform length. Similarly, the time periods associated with the dynamic segments are of uniform length, and the temperature variation is linear at the same constant rate in all dynamic or non-isothermal segments. A hypothetical isothermal baseline is calculated for each non-isothermal segment as a linear interpolation of the differential heat flow signal measured during the periods associated with the two isothermal segments which have the respective non-isothermal segment interposed therebetween. This hypothetical baseline is subtracted from the heat flow signal measured during the period of the non-isothermal segment, and the heat capacity of the material is calculated from this difference.
Another known method (H. Staub and W. Perron, Analytical Chemistry, Vol. 46, No. 1, January 1974, pp. 128-130) uses a temperature profile composed of isothermal temperature steps. The temperature difference between successive isothermal segments was decreased as the temperature was increased. Further, the length of the time period associated with the isothermal step segments was increased with increasing temperature. The peak areas defined by the heat flow signal caused in each step was used to calculate the melting enthalpy of the material in order to therefrom derive an impurity fraction of the material.
Another known method (EP 0 559 362 A1) uses a temperature profile which is a superposition of a linear temperature ramp and a periodic temperature modulation function having a predetermined modulation amplitude and frequency. Two signal components are derived from the resulting periodic heat flow signal, one of these components being related to the heat capacity of the investigated material. In this method, changes in the baseline and a resulting influence on the evaluation of the heat flow signal remain undetected.